Before anyone says it, I've already asked Magura who haven't heard much of this problem before (but they have heard of it).

Picture below shows a Magura Venti rotor mounted on my centerlock hub. You'll see the fins on the rotor protrude out over the lockring, so as you tighten the lockring the fins push the tool (a BB installation tool) off the lockring. There was just enough purchase to install it …

… but not enough to get the bloody thing off (I'm using a Shimano BB installation tool BTW).

I've thought about a tool with alternate teeth only – wouldn't work as some of the fins would still cause fouling. 4 teeth might do it but I don't know of such a tool.

Suggestions please before I get out the mole grips.

Why not take a bit of plate drilled to take 3 or 4 dowels on the right BCD for the lock ring, I did something similar once for an old BB lock ring, you only need a bit of leverage to loosen the thing so even if you only get 1/4 – 1/3 of a turn it'll be enough to do the job…

Failing that get a plastic BB tool (only about £6-7 aren't they?) and attack it with the dremel…

Venti rotors were designed for the old centre-lock, ie. the type that used a cassette tool. Now these new shimano hubs use centrelock with a bb tool the rotor basically wont work (as youve found out). Simplest way is to use a stock Magura rotor and 6-bolt adaptor. If you want to get this one working you could try filing back a small portion of the spider to flush with the locking face, this souldnt weaken it as its only that thick at the base anyway (but id do only this as little as you need to get the tool in). Or do what cookeaa says and make some sort of different tool/lockring combo. Or wait till magura release a new one that does work.

6 bolt. 😉

Sounds like you're using the wrong tool to me, you need the cassette tool with a rod on that has deeper splines.

Like this..

destruction of lockring

hammer and chisel 😉

wont be pretty ! once yu get the tension off it it will come by hand !

there are other ways but i would have to do an instructional video – i cant even begin to think how to describe it so youd understand !

Dremmel some of the rotor arms away to clear.

ziggy – thats for original centrelock, for the newer (20mm compatible) version they use a different lockring, as pictured by the OP

Lockring uses a BB installation tool, not a cassette lockring. If it did it'd be fine.

Only 20mm bolt thru uses this – the rest still use the cassette tool. Never aware that 20mm ever used the cassette tool – there wouldn't be enough room for the axle? And as I'd imagine most Ventis are on 20mm hubs it seems like a design flaw rather than a Shimano change to me.

The fins are for cooling (I think) and not strength so a dremel to them might be the best way if I can't bodge a tool…or just some 6 bolt rotors, but that's giving in.